Surface treatment of steel billets to be extruded, and of extrusion tools



United States Patent 3,135,623 SURFACE TREATMENT OF STEEL BILLETS TO BE EXTRUDED, AND OF EXTRUSION TOOLS Hubert J. Altwicker, Dayton, Ohio, assignor to Schloemann Alrtiengesellschaft, Dusseldorf, Germany Filed Feb. 24, 1961, Ser. No. 91,305 Claims priority, application Germany Apr. 5, 1960 Claims. (Cl. 117-53) In the extrusion of steel in extrusion presses, in addition to the problem of keeping the steel as far as possible free from scale, there is the problem of providing the steel with a lubricant tor its passage through the die. In general there is also the problem of protecting against scaling the tools that become heated during the extrusion.

For the purpose of obviating the scaling, the steel billets to be extruded have been heated in a protective gas. If glass is then employed, in a known manner, as a lubricant during the extrusion, it is possible after cooling, to remove the glass by mechanical means, including any iron oxide layer that may have been produced. The mechanical removal of this layer is rather troublesome, and not very advantageous, since the surface of the steel appearing after such removal is spotted, if not actually uneven owing to grooving and pitting, which have occurred through the embedding therein of grains of glass during the extrusion.

This invention obviates the abovementioned disadvantages, and consists in the use of cryoli-te, NaflAlF as the medium for the surface treatment of the steel billets and/ or of the steel tools of the press, such as the die and the mandrel for instance. The heated cryolite dissolves any iron oxide that is forming or has already formed. The layer thus produced admits of being easily washed off, after the cooling of the extruded product or of the tools that have been covered with cryolite, for instance by means of water, lye or lime milk. Cryolite, in the range of temperatures that come into question here, is more mobile or easily fusible and more pliable than glass, so that success can be achieved with a thinner coating than when glass is employed.

The liquidity and mouldability of cryolite is increased if it is mixed with a metallic oxide, for instance aluminium oxide, copper oxide, nickel oxide (particularly an oxide of a metal that is less liable to scale than iron), or with boron trioxide (B 0 and/ or with lithopone.

Use may be made of this fact, according to a further development of the invention, by first of all applying a metal, for instance aluminium, copper or nickel, in the form of copper enamel or aluminium paint or by elec trolysis, in a thin layer to the steel billet, then heating the steel billet without a protective gas, so that the metallic coating is somewhat oxidised, and immersing the billets thus treated in cryolite, or coating them with a layer of cryolite by spraying. The possibility of now heating the billets without a protective gas is of very great advantage in practice, since it renders not only the construction of the furnace but also the transport of the billets from the furnace to the press much simpler than has hitherto been the case.

The advantages enumerated admit of being obtained in approximately the same manner if the heated steel billets, instead of being treated by the process just described, are painted or sprayed with a layer of a mixture of cryolite and boron trioxide, with or without an admixture of lithopone. This mixture should preferably contain about 80% cryolite, from 5 to 15% boron trioxide, and from 15 to 5% lithopone. This mixture may alternatively be employed by painting it in around the die aperture, preferably in the angle between the die and the container wall, or in a groove in the die. This method of employment may be carried out independently of the abovementioned treatment of the billets in the heating furnace.

To explain the invention in greater detail, the following statement is made:

In the extrusion of steel it is necessary to keep the billets that are to be extruded as free :from scale as possible, and, for the protection of the extrusion equipment, to employ a suitable lubricant as a protection against the scrubbing effects of the extrusion.

The present invention starts from the [fact that the layer of iron oxide that forms during the heating of steel billets is neither coherent nor regular in its form and constitution, but is loose and porous, so that the oxidation process in question can always advance further. In practice an attempt is made to obviate this objectionable property of the steel by heating the billets in the furnace in a protective gas, by inductive heating, or by employing insulating covering means of an inorganic nature.

The present invention utilises the fact that the oxidation behaviour of some metals diliers from that of iron and steel, by coating the billets to be heated with these other metals, which form coherent and snugly adhering coatings of oxide, free from pores. Such coatings may consist for example of aluminium, copper, nickel, magnesium or zinc. They act like a natural protective sheath.

When steel billets are heated in a furnace without a protective gas, it has been found that at temperatures up to 800 C. an appreciable scaling already occurs. if a billet treated at the said temperature is allowed to cool in air, a more or less thick layer of scale falls off or becomes detached from the surface of the billet. If however the same experiment is repeated with a steel billet which is coated with a thin layer of aluminium, it is found that from this billet, when it is cooled down from 800 C. in air, no oxide layer cracks off. This of course does not mean that no oxide layer is present, but that the oxide layer is of a different nature, namely, in the case of aluminium, a thin and compact layer of alumini urn oxide.

Now a further feature of the present invention is to place the billet, for instance a billet coated with aluminium and preheated in the furnace, without protective gas, to a maximum of 800 C., in a bath of liquid salt, consisting of cryolite. This salt has a particularly high solubility for metallic oxides. If the steel billet is now heated in such a bath to the temperature of 1200 to 1250 C. which is necessary for extrusion, a dissolving process takes place on its surface, in consequence of which the coating of aluminium oxide formed during the pre-heating of the billet is dissolved, and this dissolving process leads to a very thin and thinly liquid layer of eutectic, that is, lowered melting point. The appearance of steel billets that have been supplied to the press from the cryolite bath at 1200 C. leads to the conclusion that the durability or stability of the liquid salt on the surface of the billet is very good, since such billets were completely free from scale in every respect, whereby the far-reaching covering power of this salt is demonstrated.

It is known to employ aluminium for protecting the surfaces of steel parts, a diffusion connection between a covering layer and a basic material being brought about by heating to at least 850 and up to 1000 C. In the present case a diffusion of the aluminium on the surface of the billet is undesirable; it is on the contrary necessary to convert the aluminium on the surface of the billets into aluminium oxide. This is effected by maintaining the pre-heating temperature of the steel billets at 800 C. as a maximum. With simultaneous limitation of the time of pre-heating to a maximum of two hours, the diffusion of the aluminium into the surface of the steel billet is prevented, and a very uniform coating of aluminium oxide is built up on the surface of the billet.

Experiments have shown that effects similar to those obtained with aluminium are also obtainable with copper. The latter is indeed more tolerant, in view of the diffusion, which is undesirable in the case of aluminium, of the covering metal into the basic material, because copper has a certain solubility both in alpha-iron and in gammairon, and hence the formation of hard complex compounds like Al Fe cannot occur.

For the application of the covering layers of aluminium, copper or other suitable metal it has been found advantageous to employ heat-resisting silicones as binders, and to stir the metallic powders in question into them up to such a point that a material of paint-like consistency is obtained which can be applied with a brush. The thickness of layer obtained by painting on a single coating amounts, according to experiments so far carried out, to from .01 to .005 mm. For obtaining and improving uniform adhesion it is advisable first to clean the steel billets by sand-blasting.

Cryolite has proved very effective in combination with coatings of aluminium or copper oxide produced on steel billets, when it is a question of supplying really scalefree billets for extrusion. This however is not the only problem. It is on the other hand necessary, as already mentioned, to protect the actual press tools against the stresses occurring as the glowing steel fiows through, by preventing any welding of the steel to the tool, as well as any injurious over-heating of the tool at the time of extrusion. In other words, the friction occurring in the tool must be reduced, and the flow of heat into the tool, from the steel billet that is being extruded, must be prevented.

In this connection cryolite alone has not proved completely satisfactory. This situation is changed however, if boric oxide and lithopone are added to the cryolite according to this invention. With these additions the fiowability and viscosity of cryolite are so modified and improved that mixtures of preferably 80% cryolite with to boric oxide and 15 to 5% lithopone can be employed with good success as lubricants for the press tools.

Various methods of carrying out the invention are diagrammatically illustrated in the accompanying drawings, in which:

FIGURES 1 and 2 show certain parts of an extrusion press at two successive stages of the extrusion of a billet;

FIGURE 3 is a similar view illustrating a second method of carrying out the invention; and

FIGURES 4, 5 and 6 illustrate a third method of preparing a billet for extrusion.

FIGURES 1 and 2 show a die 1, bearing against a fixed abutment 2, which has a cylindrical bore 3 for the passage of the extruded product. Against frusto-conical surfaces 4 of the die 1 a container 5 is pressed, so as to ensure a fiuidtight joint between the container and the die. The press, by means of a ram 6, forces a press disc 7 into the bore 8 of the container 5. The diameter of the press disc 7 is very accurately fitted to the diameter of the bore.

A billet 9 is heated, and is covered, in any convenient manner, with a coating 10 of cryolite. On the peripheral surface of the billet this cryolite coating is comparatively thin, but on the end face 10a it is substantially thicker.

The heated billet 9, with the cryolite coating 10, 10a, is lowered into the container 5, until it rests upon the die. Then the ram 6 advances, and presses the press disc 7 against the billet 9, so that the latter is compressed endwise, and assumes the form 9a, shown in section in FIG- URE 2, where the cryolite coatings 10 and 10a can be clearly recognised. The billet is then forced through the bore of the die 1 by means of the ram 6, thus producing an extruded product 11, which is provided with a thin cryolite coating 12, since the cryolite 10a is extruded with the steel.

FIGURE 3 shows only the container 5 and the die 1, which bears against the fixed abutment 2. By means of a paint brush or a spraying appliance a coating of cryolite is applied to the wall surface of the container bore. A thicker cryolite layer is applied to the die 1, especially in the angle between the wall surface 13 of the bore and the upper surface of the die. A hot billet, with or without a cryolite coating, is then inserted in the container 5, as in FIGURE 1, and the extrusion proceeds as in FIGURE 2.

In the example illustrated in FIGURES 4, 5 and 6, first of all a billet 20 is painted over with a thin layer 21 of a non-ferrous metal. This non-ferrous metal may be aluminium in the form of a powder, which is rendered paintable by means of a binding agent. The prepared billet 20 is heated on all sides, in air, until it has reached the desired temperature. The burners 22 may be arranged all round the billet, as indicated in FIGURE 5, or else the billet may be rotated during the heating. By this heating in air, which may be effected in an oven, the aluminum or other non-ferrous metal powder is oxidized.

The heated billet 20, with its coating of oxidized nonferrous metal, is next coated with a layer of cryolite. For this purpose the billet is clamped between a single sharp pivotal point 23 and a rotatable shaft 24 terminating in two points 25. The billet is then rotated by means of the shaft 24, and cryolite is blown on to it by means of blast nozzles 26. During this operation the cryolite combines with the already oxidized metal of the coating 21.

The billet is then ready to be placed in the press. In this case the extrusion tools (container 5, die 1 and press disc 7) do not need any special preparation, apart from being heated in the usual manner, as the coating of oxidized non-ferrous metal powder and cryolite on the billet distributes itself over the surface of the bore 8, and particularly over the face of the die 1, so that these tools are protected, besides being lubricated. During extrusion, a combined coating of oxidized non-ferrous metal and cryolite is formed upon the extruded product, as indicated in FIGURE 2.

I claim:

1. In the extrusion of steel, a ductible and easily fusible lubricant for the surface treatment of the steel billets and extrusion tools, which will prevent oxidation of the billets and of the extruded product and will lubricate the die and the internal surface of the container, and which can easily be washed off the product and the tools after extrusion, the said lubricant containing about percent cryolite, from 5 to 15 percent boron trioxide, and from 15 to 5 percent lithopone.

2. A method of applying the lubricant claimed in claim 1, by painting it, in the form of a paste, upon the internal surface of the billet-container of a press for the extrusion of steel billets, and of that surface of the extrusion die that will come into contact with the steel during extrusion, and around the die in the angle which is formed between the die and the container of the press.

3. A method of applying a lubricant to a steel billet to be extruded, consisting in first applying a non-ferrous metal coating selected from the group consisting of aluminum, copper, nickel and zinc, then heating the billet without a protective gas, so that this metallic coating oxidizes, and then applying a layer of cryolite, in which the non-ferrous metallic oxide dissolves.

4. A method of applying a lubricant as claimed in claim 3, the billets, with the non-ferrous metal coating, being heated at a temperature up to 800 C.

5. A method of applying a lubricant as claimed in claim 3, the non-ferrous metal, in the form of a powder, being mixed With a varnish, and applied to the billet with a brush.

References Cited in the file of this patent UNITED STATES PATENTS 6 Veit June 8, 1945 Henricks Mar. 4, 1952 Rossborough Sept. 30, 1958 Rossborough Sept. 30, 1958 Buehler et a1 Aug. 30, 1960 Bryant et a1 Mar. 14, 1961 Nitzsche et a1 Dec. 5, 1961 OTHER REFERENCES Part II, Procedings of the International Workshop on Information Retrival, US. Dept. of Commerce, p. 

3. A METHOD OF APPLYING A LUBRICANT TO A STEEL BILLET TO BE EXTRUDED, CONSISTING IN FIRST APPLYING A NON-FERROUS METAL COATING SELECTED FROM THE GROUP CONSISTING OF ALUMINUM, COPPER, NICKEL AND ZINC, HEATING THE BILLET WITHOUT A PROTECTIVE GAS, SO THAT THIS METALLIC COATING OXIDIZES, AND THEN APPLYING A LAYER OF CRYOLITE, IN WHICH THE NON-FERROUS METALLIC OXIDE DISSOLVES. 